The oxygen and volatile anaesthetic delivery characteristics during spontaneous ventilation using a non-rebreathing coaxial anaesthesia system.

The gas delivery characteristics during simulated spontaneous ventilation of a non-rebreathing adaptation of the Bain circuit were examined. Tidal volumes were varied from 200 to 600 ml and respiratory rate from 8 to 20 breaths per minute obtaining characteristics for a minute volume range from 2.4 to 12 litres/minute. The minimum inspired oxygen concentration with 1 litre/minute 100% oxygen fresh gas flow varied between 86 and 43% over the range of minute volumes. . The minimum inspired Sevoflurane concentration with a 4% vaporiser setting and a 1 litre/minute fresh gas flow varied between 3.4 and 1.3% over the range of minute volumes. Increasing minute volume was the primary determinant of decreasing minimum inspired concentrations with increasing respiratory rate having an independent but smaller effect in reducing concentrations. Thus this system was shown to be safe and economical for the use in spontaneous ventilation.